London, Nov. 13: For teenagers, it is a sound as irritating as their mother’s nagging: a high-pitched shriek that refuses to go away. For adults, the sound is ... silence.

It is called the Mosquito and it is designed to deter young people from loitering outside shops. It emits a piercing, high frequency sound that is audible only, in 90 per cent of cases, to people under the age of 20. While teenagers are forced to run for cover, most adults remain oblivious.

Nearly 100 stores and some local councils have placed orders for the device, which went into production on Wednesday. A retail chain also plans to introduce it in its corner shops.

The Times tested a prototype at a Hallmark newsagent’s shop in Wembley, northwest London, where staff said they were powerless to deter congregating youths.

“The worst time is when school finishes, between 2 pm and 3.30,” Gunapal Murugan, who lives above the shop, said. “They make trouble... they come and fight. You can’t do anything. They’re under age.”

At first Murugan was sceptical about the Mosquito, which he could not hear ' and was surprised by its '500 price ' but when some young customers came to the shop he watched their reactions.

“It’s like a dog whistle but I’m not a dog,” a bemused Shaker Zaman, 17, said.